<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* .triangle{
            border-bottom: 200px solid red;
            border-left: 400px solid transparent;
            width: 0px;
            height: 0px;
        } */
        .red{
            position: absolute;
            left: 0;
            top: 0;
            width: 100px;
            height: 100px;
            background: red;
            z-index: 3;
        }
        .green{
            position: absolute;
            left: 0;
            top: 0;
            width: 200px;
            height: 200px;
            background: green;
            z-index: 2;
        }
    </style>
</head>

<body>
    <!-- <div class="triangle"></div> -->
    <div class="red"></div>
    <div class="green"></div>
</body>

</html>